Mass Spectrometric Proof of Predicted Peptides: Novel Adipokinetic Hormones in Insects

Oxidation Products of Tryptophan and Proline in Adipokinetic Hormones-Artifacts or Post-Translational Modifications?

BACKGROUND

Adipokinetic hormones (AKHs) regulate important physiological processes in insects. AKHs are short peptides with blocked termini and Trp in position 8. Often, proline occupies position 6. Few post-translational modifications have been found, including hydroxyproline ([Hyp6]) and kynurenine. Our recent data suggest that the Hyp- and Kyn-containing AKHs occur more often than originally thought and we here investigate if they are natural or artifactual.

METHODS

From crude extracts of the corpora cardiaca (CC) of various insect species, AKHs were analyzed using liquid chromatography coupled to high-resolution mass spectrometry (LC-MS). Synthetic [Hyp6]-AKHs were tested in an in vivo metabolic assay. Freshly dissected Periplaneta americana and Blaberus atropos CCs (with precautions taken against oxidation) were analyzed. B. atropos CC were placed into a depolarizing saline and the released AKHs were measured.

RESULTS

Hyp was detected in several decapeptides from cockroaches. The modified form accompanied the AKH at concentrations below 7%. The [Hyp6]-AKHs of B. atropos were present in fresh CC preparations and were shown to be releasable from the CC ex vivo. Synthetic [Hyp6]-containing peptides tested positively in a hypertrehalosemic bioassay. Hydroxyprolination was also detected for Manto-CC from the termite Kalotermes flavicollis and for Tetsu-AKH of the grasshopper, Tetrix subulata. Oxidized Trp-containing forms of Nicve-AKH were found in species of the burying beetle genus Nicrophorus.

CONCLUSIONS

Trp oxidation is known to occur easily during sample handling and is likely the reason for the present findings. For hydroxyprolination, however, the experimental evidence suggests endogenous processes.

Comparative analysis of adipokinetic hormones and their receptors in Blattodea reveals novel patterns of gene evolution

Adipokinetic hormone (AKH) is a neuropeptide produced in the insect corpora cardiaca that plays an essential role in mobilising carbohydrates and lipids from the fat body to the haemolymph. AKH acts by binding to a rhodopsin-like G protein-coupled receptor (GPCR), the adipokinetic hormone receptor (AKHR). In this study, we tackle AKH ligand and receptor gene evolution as well as the evolutionary origins of AKH gene paralogues from the order Blattodea (termites and cockroaches). Phylogenetic analyses of AKH precursor sequences point to an ancient AKH gene duplication event in the common ancestor of Blaberoidea, yielding a new group of putative decapeptides. In total, 16 different AKH peptides from 90 species were obtained. Two octapeptides and seven putatively novel decapeptides are predicted for the first time. AKH receptor sequences from 18 species, spanning solitary cockroaches and subsocial wood roaches as well as lower and higher termites, were subsequently acquired using classical molecular methods and in silico approaches employing transcriptomic data. Aligned AKHR open reading frames revealed 7 highly conserved transmembrane regions, a typical arrangement for GPCRs. Phylogenetic analyses based on AKHR sequences support accepted relationships among termite, subsocial (Cryptocercus spp.) and solitary cockroach lineages to a large extent, while putative post-translational modification sites do not greatly differ between solitary and subsocial roaches and social termites. Our study provides important information not only for AKH and AKHR functional research but also for further analyses interested in their development as potential candidates for biorational pest control agents against invasive termites and cockroaches.

Predicted novel hypertrehalosaemic peptides of cockroaches are verified by mass spectrometry

Small neuropeptides from the corpora cardiaca are responsible in cockroaches for the mobilisation of trehalose from the fat body into the haemolymph. Such hypertrehalosaemic hormones (HrTHs) belong to the large family of insect adipokinetic hormones (AKHs); a few HrTHs were previously sequenced from cockroaches, and from genomic and/or transcriptomic information one may predict the genes encoding HrTHs from more species. Definite elucidation of the primary structure of the mature peptide with putative modifications needs analytical chemical methods. In the current study, we use high-resolution mass spectrometry coupled with liquid chromatography to identify unequivocally the HrTHs of 13 cockroach species. Either genomic/transcriptomic information was available for most of the species examined, or from related species. We confirm predicted novel sequences and find hydroxyproline modification for the majority of the peptides. The novel decapeptides are structurally close to Bladi-HrTH, which is found in all seven of the investigated blaberid subfamilies. Bladi-HrTH and all the novel peptides elicit a hypertrehalosaemic response in Periplaneta americana, a blattid cockroach.

The proline effect and the tryptophan immonium ion assist in de novo sequencing of adipokinetic hormones

Adipokinetic hormones (AKHs) in Arthopoda are characterized by special sequence features including limited choices of amino acid residues in certain positions, such as Trp in position 8. Over 100 different AKHs have been described, but de novo sequencing of novel peptide hormones can be a challenge. In a project of analyzing corpora cardiaca extracts from two fly species, two different moths, a termite and a beetle for their AKHs, we noted specific patterns in the fragmentation spectra of octapeptides in electrospray Q-TOF experiments resulting from the presence of Pro in position 6. The preference for cleavage N-terminal to Pro residues created an abundant y3″-ion, which, in conjunction with the two b-ions resulting from the fragmentation before and after Pro, provided a marker pattern. As Pro6 occurs in about 61% of known AKHs, this information is highly relevant for sequence elucidation. Moreover, the default presence of Trp8 allowed the use of its immonium ion for AKH candidate identification. In addition, we assembled the known AKH sequences from the literature and sequences of AKH-type format found in the Uniprot database in a single online resource. These efforts assisted in the analysis workflow and led to the assignment of two novel AKHs and evidence for the presence of Melme-CC and Dorpa-AKH in the corpus cardiacum of the scarab beetle Sinodendron cylindricum.